In video games, input lag is either the delay between the television or monitor receiving a signal and it being displayed on the screen (see display lag below), or the delay between pressing a button and seeing the game react.
In electronic hardware development, input lag is the delay between an electronic input signal being generated (pressing a button as above) and processed (IO ports have been read, and memory updated to reflect the state of the input). In this field, the phenomena detailed below, are referred to as Output Lag. More formally, the terms 'Input Latency' and 'Output Latency' are also used. Due to gamers being largely unaware of the phenomena of input lag at this low level, the terms input lag and output lag became confused over time, and the above description, where input and output lag are combined into a singular phenomena known as input lag, has become popular.
Overall, the correct terminology is clear - A delay between a physical input occurring (e.g., I/O pin voltage change) and it being processed electronically (I/O pins read by processor and memory registers updated to reflect the state of the pins) is input lag, and a delay between an electronic output being sent (e.g., memory register set to reflect the desired state of an output) and it being processed into a physically observable phenomena (e.g., memory register read and I/O pin voltage modified accordingly), is output lag. In short, Input Lag occurs on input devices, Output Lag occurs on output devices.
The following are descriptions based on the colloquial use of the term, as used by gamers.
The potential causes for "input lag"- according to the second definition- are described below (steps which have negligible contributions to the input lag have been omitted). Each step in the process increases "input lag", however the net result may be unnoticeable if the overall "input lag" is low enough.
Controller sends signal to console
For wired controllers, this lag is negligible. For wireless controllers, opinions vary as to the significance of this lag. Some people claim to notice extra lag when using a wireless controller, while other people claim that the 4-8 milliseconds of lag is negligible.
Network lag (online gaming only)
Since the game requires information on the location of other players, there is sometimes a delay as this information travels over the network. This occurs in games where the input signals are "held" for several frames (to allow time for the data to arrive at every player's console/PC) before being used to render the next frame. At 25 FPS, holding 4 frames adds 160 ms to the overall input lag. However, very few modern online games use this method. The view angle of every modern AAA shooter game is completely unaffected by network lag, for example. In addition, lag compensating code makes classification a complex issue.
Console/PC processes next frame
A videogame console or PC will send out a new frame once it has finished performing the necessary calculations to create it. The rate at which this is achieved is measured with the frame rate. Using common 60 Hz monitor as an example, the maximum theoretical frame rate is 60 FPS (frames per second), which means the minimum theoretical input lag for the overall system is 17 ms. Theoretical maximum FPS is usually limited by the video monitor, since the game cannot display more frames per second than the monitor's refresh rate (with exception of turning vertical sync (v-sync) off which in turn causes graphical artifacts). In situations where the CPU and/or GPU load is high, FPS can drop below the monitors refresh rate.
This is the lag caused by the television or monitor (which is also called "input lag" by the first definition above, but "output lag' by the second definition). Image processing (such as upscaling, 100 Hz, motion smoothing, edge smoothing) takes time and therefore adds some degree of input lag. It is generally considered that input lag of a television below 30 ms is not noticeable, discussions on gaming forums tend to agree with this value. Once the frame has been processed, the final step is the pixel response time for the pixel to display the correct colour for the new frame.
Typical overall response times
Testing has found that overall "input lag" (from controller input to display response) times of approximately 200 ms are distracting to the user. It also appears that (excluding the monitor/television display lag) 133 ms is an average response time and the most sensitive games (fighting games, first person shooters and rhythm games) achieve response times of 67 ms (excluding display lag).